1. Field of the Invention
The present invention is directed to improvements in worm - worm gear type speed reducers. More particularly, the present invention is directed to an improved worm and worm gear assembly wherein a plurality of rolling elements are mounted in the spiral teeth of the worm to provide a substantially continuous line contact between the mating teeth of the worm gear and the worm, resulting in relatively low friction and improved power transmission effectiveness.
2. Brief Description of the Prior Art
Speed reducers or torque converters having a worm engaging a worm gear are well known in the mechanical arts. As is well known, in these devices the respective axes of rotations of the worm gear and of the worm are non-intersecting and usually perpendicular to one another. Moreover, these devices are often used in heavy machinery where the transmitted power is large, and relatively large (approximately 100:1 to 20:1) speed reduction (from the input shaft of the worm to the output shaft of the worm gear) is required.
Worm - worm gear speed reducers, however, function with relatively large sliding friction between the mating teeth, so that their power transmission efficiency is low, in the range of fifty to ninety per cent (50% to 90%), usually in the lower region of the above-noted range.
Mechanical rating of these devices is defined as the power which may be transmitted through the device without excessive wear, or tooth failure, provided the device is kept reasonably cool. Thermal rating, on the other hand, is defined as the maximum power the device can transmit without overheating. Because of the relatively large friction of the worm - worm gear speed reducers, their thermal, rather than mechanical rating is usually the limiting factor in their use where large powers are transmitted and the speed reduction is large.
For the above-summarized reasons, worm - worm gear assemblies in continuous use in heavy machinery, often need to be cooled by various types of heat exchange devices, such as oil pumps, oil jets or the like. Still, the power transmission efficiency or worm - worm gear speed reducers is often less than what is desired. Moreover, because the devices have to accomodate relatively high thermal expansion of the worm, which has the high sliding velocity during operation, the differential expansion has to be accomodated in the design of the device. Overall, it is very difficult to construct worm - worm gear power transmission assemblies for high loads which operate without appreciable backlash.
The prior art has, by-and-large, recognized the above-noted serious limitations of worm - worm gear speed reducers. In order to overcome these limitations, the prior art has provided devices wherein balls are placed into the spiral thread of the worm so as to replace sliding friction with rolling friction. More particularly, U.S. Pat. No. 657,349 describes a worm having a continuous groove along the usual pitch line in its driving surface. The groove contains balls, which are said to provide "rolling" rather than "rubbing" friction with the meshing gear. The balls circulate through a reverse channel which is provided in the worm.
U.S. Pat. No. 1,347,802 describes a worm similar to the worm of U.S. Pat. No. 657,349. Balls are placed into a channel cut into one side of the spiral thread of the worm. The balls circulate through a pair of longitudinally curved tubes which are mounted to the ends of the spiral thread of the worm.
U.S. Pat. No. 3,377,879 describes still another worm having a plurality of circulating balls. The balls are placed into a helically shaped thread provided in the vicinity of the pitch line on one side of the thread, and are held therein by a ball retainer secured on the surface of the thread. Circulation of the balls is accomplished through curved guide tubes which are provided at the end of the worm.
U.S.S.R Pat. No. 427189 describes a ball worm drive where the thread of the worm is cut up by channels into individual teeth around which rows of balls circulate inside conical inserts.
Still additional disclosures relating to ball containing worms or like devices may be found in U.S. Pat. Nos. 3,494,215 and 1,885,970.
The above-noted "improved" worm - worm gear assemblies, however, also suffer from a serious disadvantage, in that placement of the balls along the pitch line of the worm fails to provide continuous, or substantially continuous line contact between the balls and the worm gear teeth which mesh with the worm.
More particularly, the theoretical facial contact between a cylindrical, helical drive worm and a mating worm gear is along a line covering the width of the worm gear tooth. This line propagates, as the worm gear rotates, from the tip of the tooth to the root of the tooth. Actually, because of virtually unavoidable machining errors, in real machine construction only approximately fifty per cent (50%) of the available worm gear tooth width is in contact, at any given time, with the spiral tooth of the drive worm.
The curvature of the contact line between the worm gear tooth and the worm thread changes continuously as the worm and worm gears rotate. This continuously changing contact line is geometrically complex, and at no time matches the circular pitch line of the worm.
For the above stated reasons, with the possible exception of the above-noted U.S.S.R. Pat. No. 427189, the ball containing worms of the prior art can have, at any given time, no more than two balls in contact with the revolving worm gear. This is because in the prior art the rows of balls in the worm are aligned either with the circular pitch line of the spiral thread of the worm, or with a likewise circular line disposed parallel with the pitch line. Thus, contact between the balls and the mating tooth of the worm gear occurs only where the "complex" theoretical contact line crosses the circular pitch line. At this "cross-over" point there can be only one, at most two balls, which are capable of transferring power from the worm to the worm gear.
Because in heavy machinery the contact pressure between the worm and worm gear is several thousand PSI, in prolonged or heavy use two balls are ill equipped to carry this load.
Stated in other words, the ball containing worm - worm gear assemblies of the prior art, which have the circulating balls disposed along the theoretical circular pitch line of the spiral thread of the worm (or along a line parallel with the circular pitch line), are, generally speaking, incapable of accomplishing their desired objective. Because of very high loads on the balls of these devices, rapid structural failure of the balls is likely, and the devices are therefore incapable of reliable use in heavy machinery. In fact, the inventor of the present invention is unaware of any commercially available ball containing worm - worm gear assembly.
The present invention overcomes the above-noted problems. The worm - worm gear assemblies of the present invention can operate with truly reduced friction under high load conditions for prolonged periods of time.